Vaso-occlusion, including stroke, is the major cause of morbidity and mortality in sickle cell disease (SS-C). It is unclear whether the enhanced pro-coagulant or platelet activity observed in SS-D directly contributes to the pathogenesis of vascular obstruction or merely reflects vascular injury caused by the sickle erythrocyte. Furthermore, while vaso-occlusion in SS-D outside the central nervous system (CNS) primarily involves microvessels, CNS vascular obstruction, or clinical stroke, involves the middle to large cerebral arteries, a likely site for platelet-induced pathology. We hypothesize that in SS-C, increased activity of coagulant proteins contributes importantly to microvascular vaso-occlusion, while platelets preferentially contribute to CNS vascular pathology. Therefore, the Specific Aims for this project are to: 1) Examine the contribution of the coagulation pathway to the development of microvascular vaso- occlusion using both genetic and pharmacologic approaches and 2) Investigate the contribution of platelets towards the evolution of sickle cell-induced CNS vascular pathology. We will use transgenic mice that exclusively express human sickle hemoglobin, together with a phenotype that closely mimics the pathobiology of severe SS-D in man, to determine the level of thrombin activation and fibrin accumulation that occurs at sites of chronic and acute microvascular disease. We will test the effect of inhibition of specific coagulation pathways using both genetic manipulation and pharmacologic therapy. Finally, we will study the role of platelets in the evolution of sickle cell-induced vascular pathology. Finally, we will study the role of platelets in the evolution of sickle cell- induced vascular pathology of the cerebral circulation using both surgically placed cranial windows to detect individual cellular events and laser Doppler flowmetry (LDF) to monitor cerebral blood flow in transgenic sickle mice. We anticipate that these studies will clarify the role of coagulant proteins in microvascular vaso-occlusion as well as the role of platelets in CNS vascular disease. Additionally, these studies will provide insights into the value of anti-coagulant and anti-platelet therapies for the treatment and prevention of vaso-occlusion.